The tenth floor: a lethal seven-heat vehicular racing arena where the last-place team dies every race.
The tenth floor, 'Don't Come in Last,' is a vehicular death-race arena. Its rules, announced by Zev over the loudspeaker: 5,501 surviving crawlers are split into 1,800 teams of up to four, running seven heats (races) that start with nine teams each. Come in last and you die — as does your garage attendant.
Each heat a different driver must take the wheel; vehicles and mounts are proprietary arena machines (mechanical or biological), 100% repaired between heats so long as the garage attendant lives. Winners pick three upgrades or one Golden upgrade; last-but-one lets the audience vote your upgrade; everyone gets a Golden upgrade after heat four. Tracks are AI-designed and change each race (the first three all paved), with shifting hazards teased in advance.
The level-collapse timer is suspended, and crawlers are ordered to 'focus on the floor like everything is normal' while armies try to vaporize the solar system.
A network of cold corrugated-metal garages opening onto AI-designed race tracks — dark asphalt roads at first, hazards changing each heat.
The setting of Book 8's first half — a survival race the party must win seven times over.